The disclosed invention is a fluid pump having an easily replaceable pumping element that is free of bearings, frictional contact or rotating parts in the flow path, and that is also free of bearings, flexing seals or apertures leading to the exterior, except for the inlet and the outlet of the pumping element. While the new pump will be advantageous in many applications where leakage, fluid contamination and damage by or to the fluid are objectionable, such as pumping pharmaceuticals and biological and sanitary fluids, a major application will be as a blood pump for circulatory assist and also for cardiopulmonary bypass.
There has been an increasing interest in continuous, rotary blood pumps for circulatory assist and circulation maintenance, to alleviate problems identified with displacement types of ventricular assist devices such as infection, thrombosis and control. There are also, however, mechanical problems with existing centrifugal (and other rotary) pumps, such as infection through a shaft seal, possible seal leakage, thrombosis and blood injury in bearings.
The fact that such problems are still prevalent with rotary blood pumps, in spite of the large number of different developments that have taken place in recent years is attested to by reference to them in a number of papers presented at an International Workshop on Rotary Blood Pumps held in Baden, Austria Sep. 9-11, 1991. It was indicated that lack of a long-lived baring is the principal obstacle to the use of a continuous rotary pump as an implanted assist device. It was also found that bearing seals in rotary blood pumps were apt to fail when immersed in blood. If there were no seal for a rotating bearing in a rotary blood pump, such problems would not exist. It was also stated that blood leakage and thrombus formation about bearing or shaft seals still limit the clinical usage of centrifugal pumps, which require seals between the blood and bearings and actuators, and that commercially available disposable blood pumps can only be used on a patient for 48 hours before being replaced. The objective is for the development of a centrifugal blood pump which can provide over two weeks of continuous operation with an inexpensive replaceable pumping element.
In the past 10 years at least two blood pumps have been developed to achieve centrifugal pumping through creation of a vortex by a planetary impeller in order to eliminate rotating bearings in the bloodstream. In one such device, disclosed in German Patent 3133177, and known as a "Teaspoon" pump, a motor-driven bent shaft rotates within an elastomeric sheath, which causes a spherical circulator (or paddle) to nutate in an annular path within the pump housing. A conventional drive motor is located in an external area, which is sealed from the blood pumping chamber by the flexible sheath. Although this device provides a potential improvement over conventional centrifugal pumps by eliminating rotation bearings and seals in the blood chamber, with the attendant dangers of infection, air embolism, thrombosis and increased hemolysis, certain limitations can be noted. There is a frictional loss due to rubbing of the bent shaft as it rotates within the flexible nutating sheath. It is indicated the sheath material is some polyurethane compound, but it is not common for a material to have good bearing and flexing qualities simultaneously. Also, it is not possible to replace the pumping element separate from the motor and drive mechanism.
In another type of nutating pump named a "Precessional Centrifugal Pump", and disclosed in U.S. Pat. Nos. 4,722,660 and 5,044,882, a circulating element in the blood pumping chamber is mounted on the end of a straight shaft which is pivoted and eccentrically driven by an external motor to provide the nutation of the circulating element. The pivot is located at the center of a flexing sealing diaphragm. The circulating nutates in a closed annular track, which is said to provide superior performance. Here, too, the flexing seal introduces a potential weakness, and in this pump, also, the pumping element itself is not replaceable without the motor and drive mechanism.
U.S. Pat. Nos. 2,107,090; 2,773,453; and 3,702,938 are older patents for nutating devices with flexing external seals that are very similar in construction to the Teaspoon and Precessional Pumps described above.
There is an evident need for an extracorporeal blood pump with a relatively long-lived, replaceable and inexpensive pumping element that has no bearings or frictional contacts within the element that could damage the blood, and no external apertures that could be paths for leakage or contamination.